The　size　effect　of　reinforced　concrete　(RC)　members　is　mainly　caused　by:　1)　the　heterogeneity　and　the　mechanical　nonlinearity　of　concrete;　2)　the　complex　mutual　effect　between　steel　rebar　and　the　surrounding　concrete.　The　use　of　high-strength　concrete　(HSC)　is　getting　increasingly　popular　as　the　structural　dimension　increases,　which　in　turn　makes　the　size　effect　behavior　more　and　more　obvious.　Moreover,　the　majority　of　RC　members　are　under　complex　loading　conditions,　which　makes　the　size　effect　more　complicated.　Experiments　on　a　total　of　16　geometrically　similar　high-strength　RC　columns　with　the　maximum　cross-sectional　size　of　800　mm×　800　mm　were　conducted.　The　mechanical　behavior　of　the　high-strength　RC　columns　subjected　to　axial　monotonic　and　repeated　cyclic　compressive　loadings　were　studied　and　the　size　effect　on　axial　compressive　strengths　was　analyzed.　The　results　indicate　that:　1)　the　failure　patterns　are　closely　associated　with　the　loading　pattern　of　the　high-strength　RC　columns;　2)　as　the　dimension　of　the　high-strength　RC　columns　increases,　the　ductility　is　getting　stronger,　however,　the　energy　dissipation　capacity　is　getting　weaker;　3)　the　nominal　compressive　strengths　of　the　high-strength　RC　columns　obtained　under　repeated　cyclic　compression　are　slightly　lower　and　have　a　smaller　discreteness　degree　than　the　ones　subjected　to　monotonic　compressive　loading,　and　the　size　effect　is　more　obvious;　4)　the　test　observations　indicate　that　the　bi-logarithmic　plot　of　nominal　compressive　strengths　for　large-scale　high-strength　RC　columns　follows　closely　the　＇size　effect　law　(SEL)＇　proposed　by　Baant.　©　2017,　Engineering　Mechanics　Press.　All　right　reserved.